1. Field of the invention
The present invention relates to a hydraulic brake apparatus for a vehicle, and more particularly to the hydraulic brake apparatus for assisting a master piston to be advanced, through an auxiliary piston by means of an assisting device.
2. Description of the Related Arts
As for hydraulic brake apparatuses, various types are known heretofore. Among them, in Japanese Patent Laid open Publication Nos.2001-334926, 2002-154422 and 2002-53028, there is proposed a hydraulic brake apparatus which is provided with a vacuum booster acting as assisting means or assisting device, and which is capable of obtaining an appropriate input-output property, even if it was failed, or after having exceeded its assisting limit. In each of the hydraulic brake apparatuses described in those Publications, there are provided a master cylinder for advancing a master piston in response to operation of a brake pedal to pressurize brake fluid in an atmospheric pressure reservoir and discharge the pressurized brake fluid, an assisting device for assisting the master piston to be advanced in response to operation of the brake pedal, and an auxiliary piston having a large diameter portion with an effective cross-sectional area larger than an effective cross-sectional area of the master piston and a small diameter portion with an effective cross-sectional area smaller than the effective cross-sectional area of the master piston, defining a first pressure transmitting chamber between the large diameter portion and the master piston, and defining a second pressure transmitting chamber between the small diameter portion and the master piston. A first valve member is provided for closing the first pressure transmitting chamber when the master piston is assisted by the assisting device through the auxiliary piston, and communicating the first pressure transmitting chamber with the atmospheric pressure reservoir, when the master piston is not assisted by the assisting device, and after the assisting device has exceeded its assisting limit. And, a second valve member is provided for normally allowing a flow of the brake fluid from the first pressure transmitting chamber to the second pressure transmitting chamber, and preventing a flow of the brake fluid from the second pressure transmitting chamber to the first pressure transmitting chamber to hold the hydraulic pressure in the second pressure transmitting chamber, and it is so constituted that the second valve member can be opened while the first valve member is being opened. According to the apparatuses as described in the Publication No. 2001-334926, the assisting piston is constituted by a single piston member, whereas the assisting piston is constituted by a couple of piston members according to the apparatuses as described in the Publication Nos. 2002-154422 and 2002-53028.
According to the apparatuses as described in those Publications, an appropriate input-output property can be obtained surely with a simple structure, after the assisting means exceeded its assisting limit, and when it was failed. According to the apparatus as described in the Publication No. 2002-154422, a master piston can be reduced in length along its longitudinal axis, and easily manufactured at a low cost. According to any of the hydraulic brake apparatuses, however, a pedal feeling can not necessarily be improved satisfactorily, it is so constituted that after the brake pedal is released, the second valve member is opened with the pressure in the first pressure (transmitting) chamber being reduced. Therefore, the pressure holding state in the second pressure (transmitting) chamber is canceled, whereby the auxiliary piston is immediately retracted to its initial position, or the small diameter portion of the piston is immediately retracted until it abuts on the large diameter portion of the piston in the Publication Nos. 2002-154422 and 2002-53028. In those hydraulic brake apparatuses, a valve opening mechanism including the second valve member is of a type responsive to the hydraulic pressure, so as to operate in response to variation of the hydraulic pressure, respectively, improvements in pedal feeling and simplicity in structure have been required.